Nitronates Preparations cyclic-five-membered

In some cases, five-membered cyclic nitronates can be prepared by the chemos-elective replacement of one of two different halogen atoms in 1,3-dihalopropanes... 

In the synthesis of five-membered cyclic nitronates (5), the problem of stereoselectivity is in preparing these products with desired relative configurations of the stereocenters at the C-4 and C-5 atoms (see Scheme 3.12). Generally, the trans configuration of these substituents is most preferable. Several procedures giving exclusively this configuration were documented (see,e.g. (50, 55, 58, 63, 68)). 

Synthesis of Five-membered Cyclic Nitronates from a-Halogen-substituted AN. The key step of this approach is presented in Scheme 3.1, path (b). Until recently, this synthetic route to nitronates (24) has been of no preparative interest, because only two examples, such as elimination of trimethylsilyl nitrite (75) and 1,2-dinitrophenylethane (85) from the corresponding nitroso acetals were documented. 

In addition, chiral five-membered cyclic nitronates can be prepared from optically inactive starting nitronates with the use of ligated palladium (catalyst) as a chiral inductor (71) (ee 97%). 

Elsewhere, Heaney et al. (313-315) found that alkenyloximes (e.g., 285), may react in a number of ways including formation of cyclic nitrones by the 1,3-APT reaction (Scheme 1.60). The benzodiazepinone nitrones (286) formed by the intramolecular 1,3-APT will undergo an intermolecular dipolar cycloaddition reaction with an external dipolarophile to afford five,seven,six-membered tricyclic adducts (287). Alternatively, the oximes may equilibrate to the corresponding N—H nitrones (288) and undergo intramolecular cycloaddition with the alkenyl function to afford five,six,six-membered tricyclic isoxazolidine adducts (289, R = H see also Section 1.11.2). In the presence of an electron-deficient alkene such as methyl vinyl ketone, the nitrogen of oxime 285 may be alkylated via the acyclic version of the 1,3-APT reaction and thus afford the N-alkylated nitrone 290 and the corresponding adduct 291. In more recent work, they prepared the related pyrimidodiazepine N-oxides by oxime-alkene cyclization for subsequent cycloaddition reactions (316). Related nitrones have been prepared by a number of workers by the more familiar route of condensation with alkylhydroxylamines (Scheme 1.67, Section 1.11.3). 

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